The lover of Alpines is so much the better placed in one respect than his brother amateur who specializes in herbaceous plants that he has some vegetation, and not infrequently bloom, to show even in the winter months. The rock gardener is generally weakest when the herbaceous lover is strongest - from July to October inclusive.
In every department of flower gardening there ought to be prolonged interest. In many cases specialistic plant growing is not gardening, because it merely aims at making particular plants grow, and does not consider them in connection with other subjects, or with the garden generally. Every plant that is put into the ground ought to be planted with these things in mind.
Shrubs should be considered for the rock garden: (1) because they enter into harmonious relations with other occupants; (2) because they serve as permanent furniture for the garden; (3) because they give beauty of bloom or foliage at a period when the majority of Alpines are ineffectual. The smaller Rhododendrons, the Cistuses, and the Helianthemums - to mention only three genera - will provide bloom practically throughout the summer, and there will be no period of the year when they are other than ornamental.
Fig. Linum Arboreum. A Beautiful And Useful Shrub For The Rock Garden. Photo by P. A. Malby.
The rock-gardener is apt to overlook shrubs because they are associated in his mind with bulky blocks of uninteresting evergreens. He does not realize the wide range of shrub-habit and beauty. He does not consider that many shrubs are truly Alpine in character, and are never better placed than among stones. The common coarse evergreens have nothing in common with these subjects, except the mere physical fact that they bear their stems throughout the year.
In a garden that is planned with real art the shrub and rock gardens will be so interwoven that they cannot be separated. Groups of shrubs will be planted at the approaches to the garden, and also on the mounds within it. They will be in character with their surroundings. They will bear a proportion in size and habit to the conformation of the ground and the mounds and stones.
We may very well consider some of the shrubs best suited to the purpose in view.
Visitors to Switzerland will be familiar with the Alpine Rose, Rhododendron ferrugineum, coloured figures of which appear in the sister volume, "Alpine Flowers and Rock Gardens." This beautiful little evergreen grows about a foot high, and has foliage resembling Box. It bears small umbels of tubular rosy flowers. There is a white form called alba or albiflorum.
Another Alpine Rose is R. hirsutum, which is likewise a dwarf evergreen, with hairy leaves, and red, funnel-shaped flowers in umbels. A coloured plate appears in "Alpine Flowers and Rock Gardens," showing the plant growing below Pinus Cembra, the bright umbels of bloom admirably displayed by the grey-green foliage of the Pine. There is a white form.
Fig. Use of shrubs in the rock garden. Showing how a rough bank in the author's garden was treated by forming a pool, setting steps and stones, and planting small shrubs.
There is considerable interest in R. intricatum, an evergreen species introduced from Yunnan by Wilson, as recently as 1907. It has very small scaly leaves on much crowded branches. It is often seen bearing its small mauve flowers when no more than six inches high, and is not likely to grow over two feet. It is not, however, hardy enough to be planted in cold districts.
Another modern species from China and happily more hardy than intricatum, is R. flavidum (primulinum) a dwarf evergreen with Myrtle-like leaves and pale yellow, bell-shaped flowers in clusters. This plant must not be confused with R. flavum (Azalea pontica).
R. Przewalskii is a Chinese evergreen species, compact in habit, with white bell-shaped flowers.
R. racemosum is a deciduous shrub from Yunnan, dwarf and compact, with white or pale rose flowers in clusters along the branches.
Among the hybrid Rhododendrons Early Gem is one of the most suitable, for it is dwarf and blooms freely. The colour is rosy lilac. This plant is often forced in pots. R. compactum multiflorum could be planted. R. arbutifolium, myrtifolium, and Wilsonianum, all with rose flowers, are other dwarf hybrids well suited to the rock garden.
The shrub-lover whose soil is free from lime should endeavour to plant a selection of these dwarf Rhododendrons in his rock garden. They will serve admirably for the upper parts of rock beds. Although they love peaty soil they must not be planted in wet, undrained bog, as they dislike stagnant moisture at the roots.
The Heaths and species of Erica will serve the same purpose as Rhododendrons and thrive under similar conditions - that is, in peaty soil on elevated, well-drained sites. In districts where the natural soil is peaty Heaths should be planted freely on the upper part of sloping ground which is set with stones for alpines. Sharply-sloping ground forms a natural place for rock gardening, as with stones partially embedded in tiers to give an approach to the appearance which stones have when lying in their natural strata all semblance of artifice is done away with. Ericas carnea, ciliaris, codonodes and mediterranea may all be planted; the last two will sometimes tolerate lime. E. carnea forms a dense carpet and is beautiful in early summer.